Evolutionary changes and genetic variation can drive ecological dynamics over short timescales. However, few studies has explored whether the phenomenon actually occurs in natural ecosystems. In this study, we conducted a large-scale field experiment to examine how genetic variation in adaptive traits of an herbivorous insect, Plagiodera versicolora, affect its population dynamics, and whether the variation impacts on shaping tree-associated arthropod communities. Plagiodera beetle has between-population genetic variation in feeding preference for leaf-age types and we found genetic correlation between the preference and predator avoidance behavior. In July 2014, we set up tree-canopy enclosures of mature willows and released genetically distinct Plagiodera populations with following treatments: single-source populations and mixed-source populations, respectively. Our results showed that population dynamics of Plagiodera beetle was significantly different among treatments. Furthermore, arthropod community structure was also affected by the genetic variation in Plagiodera.